In-depth technical analysis of 802.11ac

I. Overview

Most 802.11n devices are designed for the GHz band, with fewer available channels and other devices working in the GHz band (such as Bluetooth, microwave oven, and wireless surveillance cameras) even if the connection speed can reach 300 Mbps under the 40 MHz bandwidth of the dual-space stream, the actual throughput is not high in the actual network environment due to mutual channel conflicts and other reasons, poor user experience. 802.11ac is specially designed for 5 GHz frequencies. It features new RF features and can improve the performance throughput of existing wireless LAN to a level comparable to that of Wired Gigabit Networks.

As a new standard of IEEE wireless technology, 802.11ac draws on the advantages of 802.11n and further optimizes it. In addition to the most obvious high-throughput features, it not only can be well compatible with 802.11a/n devices, at the same time, it also improves the user experience.

In the 802.11ac network, each Wireless Access Point can accept more clients, provide more bandwidth for each parallel business flow, and reduce latency and power consumption.

Ii. Major Technical Features of 802.11ac

Because 802.11n is already excellent on the MAC layer, there are not many improvements to 802.11ac on the MAC layer. It mainly uses the PHY layer to increase its basic speed.

Improvement on the PHY Layer

1. More secure modulation mode

802.11ac continues to adopt the OFDM modulation method in 802.11a, but increases the order from the 64-level in 802.11n to the 256-level. 256-QAM makes the number of data bits for each sub-carrier increase by nearly 33% from 6 to 8. Because 256-QAM is more sensitive to interference and suitable for environments with high signal-to-noise ratio, the 256-order orthogonal amplitude modulation is mainly used in the range where the 64-order orthogonal amplitude modulation has been reliably covered. Although 256-QAM provides a higher rate, it does not increase the effective coverage distance.

Exceptions: Unlike 802.11n, non-classified constellation modulation can be used. In 802.11ac, when multiple streams are sent at the same time, each stream must use a constellation of the same size. The former is very beneficial for multi-space streams, especially beam forming.

2. Wider channel bandwidth

Because the 5 GHz band provides more channels and wider bandwidth, 802.11ac increases the channel bandwidth from 20 MHZ and 40 MHZ of 802.11n to 80 MHZ or even 160 MHZ.

The increase in bandwidth brings about an increase in available data subcarriers. The number of available sub-carriers at 80 MHz reaches 234, while that at 40 MHz is only 108, so that 80 MHz can increase by 2.16 times. The minor side effect is that the same transmission power needs to be separated to multiple sub-carriers, which will slightly reduce the signal coverage, but in general it is better.

The increase in bandwidth brings about the difficulty of Channelization. However, 802.11ac still follows the simple and effective method of 802.11n. Just as 802.11n combines two adjacent 20 MHz into 40 MHz, 80 MHz is combined through adjacent 40 MHz. The 80 MHz must be merged using the adjacent 40 MHz, and there is no overlap between the 80 MHz. Because the number of MHz obtained through the continuous 80 MHz combination is very small, the MHz can be obtained using the discontinuous 80 MHz, that is, the 80 + 80 mode. Channelization 1:

Figure 1 Channelization

In 40 MHz bandwidth, there are two main channels and secondary channels (that is, the second 20 MHz channel), so there are still differences in 80 MHz. In a channel with a bandwidth of 80 MHz, a 20 MHz channel must be selected as the primary channel. In the channel with a bandwidth of 40 MHz, the remaining 20 MHz channel is called the secondary (second) 20 MHz channel, the 40 MHz channel that does not contain this primary channel is called the secondary (second) 40 MHz channel. 2.

Does the increase in channel bandwidth mean that less available channels conflict with more channels? Actually, it won't. In the 5G band, China has already opened 149,153,157,161 and 165 frequencies. Although only one 80 MHZ band can be deployed at present, the existing mechanism allows two devices to be deployed at the same 80 MHZ at the same time, one of them deploys its main channel at a low bandwidth of 40 MHz, and the other at a high bandwidth of 40 MHz, even if the two transmit at the same time, there is still no conflicting 40 MHz bandwidth available. This is the same as the existing 802.11n deployed at 40 MHz bandwidth.

Figure 2 channel naming

In addition, for transmission at a bandwidth of 80 MHz, the enhanced RTS/CTS mechanism of 802.11ac can effectively coordinate the channel usage between 802.11ac and 802.11a/n devices. When the master channel of 802.11a/n is in the 80 MHz bandwidth of the 802.11ac deployment, if the overlapping part is not in the 40 MHz bandwidth of the master 20 MHz, the 802.11ac can be dynamically downgraded to the 40 MHz mode, the throughput corresponding to the 40 MHz bandwidth is obtained, as shown in 3 (a). If the overlapping part contains the master 40 MHz bandwidth of 20 MHz, the 802.11ac and 802.11a/n obtain the right to use the channel through competition, assuming that each of the 50% opportunities is occupied, 802.11ac will use 80 MHz for sending for half of the time. Therefore, the half throughput is the same as the throughput obtained in Figure 3 (.

Therefore, increasing the channel bandwidth does not cause less available channels to conflict with more channels.

3. More space streams

In 802.11n, a maximum of four spatial streams are supported, while 802.11ac increases the upper limit to eight. This alone doubles throughput. In single-user transmission, the MCS of each stream is the same. In the new MU-MIMO technology, each STA can use up to 4 streams, and each stream MCS must be the same for all users.

The above three points allow the rate of 802.11ac to reach up to 6.9 Gbps. The following is a comparative analysis of the RF parameters and rate of 802.11a/n/ac:

4. Wave Velocity Forming

Any device that uses multiple antennas can form the velocity of any other device at any time. 802.11ac defines a test protocol (VHT Sounding protocol ). This Protocol gives the acceptor the opportunity to help send data to better form the velocity.

This protocol requires Beamformer (wave velocity forming sender) to initialize the wave velocity forming sequence by sending NDPA (empty packet announcement. In NDPA, Beamformer adds the STA information field to each Beamformee (wave speed forming receiver) in NDPA, and sets the AID information of the corresponding STA in this STA information field, this is to enable each Beamformee to prepare a VHT beam frame to be compressed. An NDPA frame must contain at least one STA information field. The VHT-NDP packet will be sent with NDPA, and only one SIFS is separated in the middle. Except for the SIFS + VHT-NDP frame after NDPA, it cannot be other frames.

If NDPA contains more than one STA field, NDPA must be sent in broadcast mode, that is, RA (Receiver Address) must be a broadcast Address; otherwise, it is sent in Unicast mode, and RA is the Receiver Address.

Each predefined receiver uses the vht ndp leading character to measure the RF channel from the Wireless Access Point to itself and compress the channel. The first pre-Receiver immediately uses the compressed channel information in the frame formed by the VHT compression speed to respond, and other pre-recipients wait for the round-robin response. Figure 9-41a shows the velocity of a single user, and Figure 9-41B shows the velocity of a multi-user. In addition, 802.11ac is not compatible with 802.11n.

5. MU-MIMO

802.11ac proposes a new technology, that is, multi-user multi-input output (MU-MIMO ). Compared to the 802.11n device, multiple spatial streams can only be sent to a single user at the same time, this MU-MIMO technology means that in the 802.11ac network, multiple users can receive at the same time, if the throughput of a single user is Mbps, the total throughput of multiple users can reach 1 Gbps. 802.11ac becomes a small switch in the wireless network.

MU-MIMO technology will be a very challenging technology in real deployment. In the demonstration shown in figure 4, in order to send a strong wave speed (blue) to user 1, the wireless access point needs to be in the other two users (user 2 and user 3) to reduce the energy of user 1. This blank operation is displayed as a blue concave port. Similarly, when sending messages to user 2, you need to reduce the energy of user 2 in user 1 and user 3. In this way, we can achieve the strong signal needed by the corresponding user, and reduce interference to other users. This method requires the wireless access point to know the channel information from itself to each user. Therefore, the Wireless Access Point must continuously detect the channel and increase the overhead. At the same time, the signal received by the user will be mixed with the interference of the signal sent to other users, making it impossible to reach the maximum modulation mode, especially 256-QAM will become unsuitable.

Fig. 8 MU-MIMO

MAC Layer

1. A-MPDU

802.11ac defines that each 802.11ac frame is a A-MPDU frame, even if this A-MPDU only contains 1 MPDU. One A-MPDU of 802.11ac can be up to 1 MB (1048575 ETS), while 802.11n is only 64 KB (65535 octets ).

2. RTS/CTS

In 802.11n, RTS/CTS undertakes the cleaning task, so that the 802.11a/g device stops transmission during its transmission to avoid conflict. In 802.11ac, because 80 MHz uses more channels, it is necessary to improve the RTS/CTS mechanism to deal with communication conflicts on the secondary channel. After improvement, the "Dynamic Bandwidth" mode is also supported by the RTS/CTS.

Before transmitting 802.11ac, You need to monitor whether the channel is available. When available, the 802.11ac device sends the RTS over the 80 MHz channel in use. This RTS uses the 802.11a frame format, which is transmitted over every 20 MHz channel (first transmitted on the primary channel, and three copies are copied at the same time to fill the entire channel, or copy 7 copies to fill the entire MHz channel), and the RTS carries bandwidth information. All the 802.11a/n/ac devices of the primary channel in this 80 MHz can receive and parse this RTS. When receiving this RTS, the receiving end will determine whether all 20 MHz channels are available and whether nearby devices are using these channels. Then, based on this judgment result, replies to CTS over available channels and reports available bandwidths in CTS. Finally, the sender sends data over these available bandwidths. These available bandwidths must contain primary channels.

"Dynamic Bandwidth"-if the receiver finds that some channels are very busy, it can notify the sender not to use these channels, and the sender dynamically drops back to the low-level bandwidth mode.

802.11ac also removes some things that are not very useful in 802.11n. Because it is VHT, there are naturally more things in the wireless frame structure that express VHT information, therefore, the frame structure will be changed. In addition to the changes, 802.11ac adds two mac frames: NDPA and Beamforming report poll.